It does depend very much upon the profile of whatever tyres you are running, but for general 4 wheeling stuff on 55 or higher profile tyres, 15psi or so is probably about as low as you'll want to go to avoid damaging rims &/or tyres; just please DO remember to be gentle with your steering, braking, and other control inputs.

Buuut, if you come across some particularly soft sand or a steep loose hill, you can probably drop them lower for a short period to clear a particular obstacle just so long as you avoid wheelspin & any aggressive control input - ie, Big Red, mid arvo, really hot day, drive up to the bottom of the dune, stop, drop your tyres to 8psi, select low range second gear, and you'll pretty much idle over from a slow standing start at the bottom, just adding enough right foot to avoid letting it bog down. Then put them back up a tad for the 'normal' 4WD'ing stuff. Works for me, EVERY time!

An Ex-Service person is someone who thought enough about their country & how great it is, how lucky we are to live here, to write a blank cheque made out to 'The People and Commonwealth of Australia' for the value of 'Up to & including my Life!'